The present invention relates generally to the precision machining of parts, and more particularly to a device which compensates for the thermal sensitivity of a precision machining apparatus to allow machining at higher levels of accuracy.
Reflective optics, infrared lenses and domes, spheric and aspheric lenses, fine ceramics, precision molds and other complex mechanical components are just examples of articles which, because of their specific applications and uses, require close tolerances in all dimensions. Therefore, the machining of such articles must be conducted on ultraprecision machining devices. Highly advanced multi-axis devices are commonly employed to machine such articles since they provide a very high degree of the requisite precision. These devices include lathes having a movable slide which carries a spindle and a slide which carries a toolholder for movement in a direction parallel to the longitudinal axis of the spindle. Single-crystal diamond, cubic boron nitride or carbide cutting tools are normally utilized to cut the workpiece mounted on the spindle. A single-crystal natural diamond cutting tool is preferred for removing materials such as non-ferrous metals, polymers and crystals because of its resistance to wear, the quality of its edge (which minimizes sub-surface stress and damage), its low coefficient of friction, chemical inertness, high thermal conductivity and low coefficient of thermal expansion. Lathes of this type are often referred to as diamond tool lathes and an article machined on a diamond tool lathe is said to have been diamond machined.
Those skilled in the art recognize that the machining of most symmetric and asymmetric components on a lathe requires the utmost precision in locating the center of the workpiece (which should be in the longitudinal axis of the spindle and chuck which holds the workpiece). Most applications of diamond tool lathes are in the field of optics where even a slight deviation from the center of the workpiece might render the component useless. Typically, numerically controlled lathes utilize resolvers, encoders, interferometers or linear scales to furnish position feedback information to the controller which positions the slides of the device. Such feedback information would be utilized to locate and continuously monitor the center of the workpiece being machined. In the production of symmetric and asymmetric components, the diamond cutting tool of the lathe would be moved from the perimeter of the component to the center of the workpiece as the workpiece is rotated by the spindle. Such sweeps of the tool against the workpiece would be made successively to remove material until the desired contour of the component was attained. It is imperative, therefore, that the exact center of the workpiece be located on each sweep.
Most prior art arrangements for locating the center of the workpiece are installed on diamond tool lathes without regard to the fluctuating temperature in the environment surrounding the lathe or the temperature of the lathe components themselves. It is appreciated, but not fully understood, that temperature has a significant influence on the accuracy of machining and measurement. The thermal growth of the lathe components occurs for several reasons including heat generated by motors, moving components, bearings and friction. Since many lathes are adapted to move distances of approximately 0.000,001 inches, any significant thermal expansion of the lathe components, such as the slides or the supporting structure for the slides, effects the accuracy of positioning the cutting tool or locating the center of the workpiece. Thus, as the lathe components expand due to a rise in temperature in and about the lathe components, the sweeps of the tool to cut the surface become successively less accurate.
The few attempts in the industry to compensate for the thermal sensitivity of ultra-precision machines such as diamond tool lathes have included the use of metrology frames. Metrology frames carry only position-sensing equipment and are built around the machine itself to isolate the position-sensing equipment from the load-carrying members of the machine so that the changing loads on the load-carrying members of the machine do not affect the position-sensing equipment and the position measurements. Typically, metrology frames employ laser interferometers or other feedback devices to furnish the requisite position information. This position information is obtained by measurements made behind and to the side of the workpiece and cutting tool. The points from which position measurements are to be made are chosen without regard to the workface of the workpiece or the tip of the cutting tool. Thus, the manner in which metrology frames "link" the two axes of a two-axis device takes into account only the center of the spindle and workpiece. With respect to thermal sensitivity, the servo motors, laser interferometer and other components of metrology frames are thermally isolated from the machine components in an attempt to reduce or eliminate position measuring error introduced by the thermal expansion of such machine components. Some metrology frames are made from alloys having relatively low coefficients of expansion in an attempt to minimize the effect of temperature changes. However, the cost of materials having low coefficients of expansion is high, therefore inhibiting their use in most commercial machines. Further, the advantages of using alloys having relatively low coefficients of thermal expansion is minimized because of the manner in which the metrology frame links the axes of ultra-precision machines. In prior art metrology frames, there is no attempt to make all displacement measurements from a common, specifically defined axis. Thus, the position information obtained from the measurements made on a metrology frame system is limited.
Experience has revealed that metrology frames can provide acceptable results in the initial stages of operation after the position-sensing equipment has been properly aligned and adjusted. However, once the components of the ultra-precision machine employing a metrology frame undergo a rise in temperature, the alignment and adjustment of the position-sensing equipment is detrimentally affected. Therefore, ultra-precision machines employing metrology frames must frequently, as much as two to three times in a day, be taken out of operation so that the position-sensing equipment can be realigned and readjusted with respect to the center of the workpiece. Further, the only way to adjust and align such a device (i.e., set to center) is by a trial and error technique. Specifically, such a technique would require one to make a cut on a mock workpiece, analyze the cut workpiece on an interferometer, adjust the device, and repeat these steps until the interferometer shows the device to be properly aligned to center. This, of course, is tedious, expensive and slows production of the article being manufactured. Other drawbacks of metrology frames include their expense, their intricate installation, and their bulky size.
The foregoing demonstrates that a device which substantially minimizes the thermal sensitivity of lathe components so that accurate position measurements can be readily taken throughout the machining of an article is warranted. Such a device should link the axes of a two-axis system to take into account the dimensions being measured with respect to the cutting tool and the workpiece and how such dimensions are being measured. It is also desirable that existing machinery could be easily modified with such a device.